A touch screen or touch pad is generally based on a two layer solution or single layer solution. With the two layer solution, rows and columns of conductive cells are interconnected. Thus, when a user touches the screen, a capacitor bridging a row and a column affected by the touch is formed. Accordingly, the two layer solution is called a bridging capacitor screen, mutual capacitor screen or projected capacitance screen. To make the arrays of rows and columns to work, the two layers need to be perfectly aligned during the manufacturing, which is a complex task. In addition, large numbers of connections need to be made as well, thus adding to extra manufacturing cost.
In the single layer solution called a surface capacitance screen, a continuous sheet of conductive material is deposited on glass or film. The sheet is used as a resistor. There are also electrodes at the four corners of the screen. Thus, when a user touches one spot of the screen, the screen acts as a two dimensional resistive divider. When electrical charges are accumulated in or removed from a capacitor formed between the spot and the user's finger, the current will flow to each electrode proportional to the distance of the user's finger from the electrode of the panel.
To calculate the horizontal position of the surface capacitance of the screen, the top and bottom electrodes of one side and the top and bottom electrodes of the opposite side are bridged, turning the screen into a resistor in one direction. Then, the capacitor formed between the user's finger and the position on the panel is charged up. As the capacitor is discharged, currents flowing through two different resistors formed when the screen is divided by the capacitor are measured. The two currents are subtracted and divided by the sum of them to make the value insensitive to the capacitance. The ratio would provide the x-coordinate of the position of the user's finger on the screen. The y-coordinate of the finger may be obtained by repeating the same process with the left and right electrodes of the top side and the left and right electrodes of the bottom side are bridged.
However, the surface capacitance of the panel is inherently non-linear. Since the voltage source is only connected to electrodes present at the corners, the amount of voltage applied to the top and bottom or the left and right across the screen may not be even. To compensate for the voltage drop across the screen, the electrodes are formed according to a linearization pattern. In the pattern, the distance between the electrodes facing each other across the screen becomes shorter as the electrodes are closer to the center of the screen, whereas the distance becomes longer as the electrodes are closer to the edge of the screen. That is, the voltage drop is compensated by lowering the resistance between the electrodes. However, the pattern consumes extra space and the sensitivity of the screen suffers as the resistance toward the center of the screen is lowered. Therefore, in this design as the linearity is increased, the sensitivity is decreased because the resistance is lowered. Furthermore, the surface capacitance of the screen is limited to determining the position of a single touch.